Activated carbon for use in electric double layer capacitors

ABSTRACT

Activated carbon adapted for electric double layer capacitors is provided, which capacitors can give a large power density per unit volume and which capacitors, even when charge-and-discharge cycle is repeated under a large current or a constant voltage is continuously applied for a long time, undergo less decrease in output density. 
     That is, the present invention relates to the activated carbon manufactured by carbonization of coconut shell, which has a BET specific surface area of 2000 m 2 /g to 2500 m 2 /g, an average pore diameter of 1.95 nm (19.5 Å) to 2.20 nm (22 Å) and a pore volume of pores having a pore diameter calculated according to a Cranston-Inkley method of 5.0 nm (50 Å) to 30.0 nm (300 Å), of 0.05 cm 3 /g to 0.15 cm 3 /g.

FIELD OF THE INVENTION

This invention relates to activated carbon for used in electric doublelayer capacitors and, more particularly, to activated carbon for use inelectric double layer capacitors manufactured from coconut shell andhaving excellent performance of a high output density and an excellentdurability.

DESCRIPTION OF THE BACKGROUND

In recent years, in view of global environment protection, it has beenconducted in the field of automobiles to develop technology for moreimproving specific fuel consumption and more purifying exhaust gas. As apart of such development, technology development on hybrid vehicles andelectric vehicles have been promoted. With respect to the development ofthese technologies, practicable electric double layer capacitors haveattracted attention for the use of driving system power assist or energyregeneration. Principle of the electric double layer capacitors is basedon accumulation of charge between electric double layers formed at theinterface region between a polarizable electrode and an electrolyticsolution, advantage of which is that, in comparison with secondarybatteries such as lead storage battery and nickel hydrogen secondarybattery, they permit rapid charge and discharge at a large current. As amaterial for the polarizable electrode, activated carbon is commonlyused due to its large interface and excellent conductivity. Electricdouble layer capacitors demanded for the use of hybrid vehicles andelectric vehicles are required to have not only an electrostaticcapacity (F) but a high power (W) density permitting repeatedcharge-and-discharge at a large current of 100 A order as well, and manytechnical problems still remain unsolved. Thus, in order to increase thepower density, there have been proposed such electrode-manufacturingtechniques as that thickness of porous electrodes mainly comprisingactivated carbon, thickness of current collectors and thickness ofseparators are made adequate (Japanese Patent Laid-Open No. 317332/1999)and that a composite electrode of activated carbon impregnated withaluminum is made as a polarizable electrode (International PatentLaid-Open No. 509560/1998).

On the other hand, it has been disclosed to use a carbonaceous materialrepresented by activated carbon as an electrode material for electricdouble layer capacitors in Japanese Patent Publication No. 15138/1985,Japanese Patent Laid-Open Nos. 187614/88, 321620/1989 and 180013/1991,Japanese Patent Publication Nos. 56827/1994, 44407/1992 and 70770/1992.

Japanese Patent Laid-Open No. 321620/1989 discloses to use powderyactivated carbon having been heat-treated at 1000° C. as an electrodematerial for capacitors, and Japanese Patent Laid-Open No. 180013/1991discloses that electrostatic capacity of an electric double layercapacitor can be improved by adjusting amount of oxygen contained inactivated carbon to from 25 to 35% by weight. In both publications,however, kinds and physical properties of the activated carbon are notdisclosed at all.

In Japanese Patent Publication No. 70770/1992, it is disclosed thatcapacitors using activated carbon having an average pore diameter of 1.5nm (15 Å) or more as an electrode material for the capacitor showexcellent temperature properties and, in Japanese Patent Laid-Open No.187614/1988, it is disclosed that powdery activated carbon showing ahigh electrostatic capacity has a specific surface area of 1800 to 3500m²/g and an average pore diameter of 0.5 to 1.5 nm (5 to 15 Å), with theratio of pore volume of pores having a diameter of 2.0 nm (20 Å) or moreto the total pore volume being 20 to 40%. In these publications,however, it is not disclosed to select a specific range of 1.95 nm (19.5Å) to 2.20 nm (22 Å) as to average pore diameter and select a range of0.05 cm³/g to 0.15 cm³/g as to pore volume of pores having a porediameter of 5.0 nm (50 Å) to 30.0 nm (300 Å), as are specificed in theinvention.

With respect to fibrous carbons, Japanese Patent Publication No.15138/1985 discloses that fibrous carbon obtained by carbonizingphenolic fibers and conducting activation treatment is suited as anelectrode material for capacitors and, in Japanese Patent PublicationNo. 56827/1994, it is disclosed that carbon fibers or carbon powderhaving a surface acidic functional group at a concentration of 1.0meq./g or less is used as an electrode material for capacitors, thatthose with a specific surface area of 500 m²/g or more are preferred,and that phenol resin-based activated carbon fibers are specificallyused. In addition, Japanese Patent Publication No. 44407/1992 disclosesto use a product obtained by carbonizing and activating a specificphenol resin foam as an electrode material for capacitors.

However, although activated carbons obtained by carbonizing andactivation-treating phenol resin-based materials as disclosed inJapanese Patent Publication Nos. 15138/1985, 56827/1994 and 44407/1992have about the same specific area as that in the invention but, sincethe phenol resin-based carbonization products to be activated are ofgenerally amorphous carbon, most of pores formed in the activated carbonobtained by activating the carbonization products have a pore diameterof 15 Å or less. Hence, as will be shown in Comparative Example 4 to begiven hereinafter, the activated carbon usually has an average porediameter of 18 Å or less which is smaller than the average pore diameterin the invention, and have substantially no pore distribution in acomparatively large pore region of 5.0 to 30.0 nm.

Besides, since electrode materials for electric double layer capacitorsdisclosed in these publications are intended to increase electrostaticcapacity per unit weight or per unit volume (F/g or F/cm³), dischargecurrent density per unit area of the electrode shown in the charging anddischarging test conducted in the specific examples is at a considerablylow level of about 0.1 to about 2 mA/cm², and effects as electrodematerial at a high current density of, for example, 10 mA/cm² or moreare not confirmed at all, thus these publications not disclosingactivated carbons suited for the high power density uses at all.Activated carbon-fiber woven cloth prepared from phenol resins,disclosed in Japanese Patent Publication No. 15138/1985, has anadvantage that it has a smaller electric resistance than electrodesobtained by molding or coating powdery activated carbon and, in thissense, it is preferred because a high power density is expected.However, it has the defect that capacity per volume is small thoughoutput per weight is large due to the small bulk density of theelectrode made of it.

Accordingly, the electric double layer capacitors using as electrodematerial the activated carbons disclosed in these publications are notnecessarily said to give a large output density per unit volume, thoughthey show an increased energy density due to a comparatively largeelectrostatic capacity thereof.

Further, advantages required for the electric double layer capacitorsare that number of usable charge-and-discharge cycles be large and thatthey have a high durability such as that they undergo a smaller decreasein capacity in a continuously voltage-applying test, as compared withthose of the conventional secondary batteries. The electric double layercapacitor using, as an electrode material, activated carbon containing alarge amount of oxygen as disclosed in the foregoing Japanese PatentLaid-Open No. 180013/1991 suffers a serious decrease in capacity due toan increase in resistance or the like upon charging and discharging,thus involving a serious problem as to durability.

With electrode materials using the known activated carbons describedhereinbefore, it is possible to decrease inner electrical resistance ofthe electrode and increase the power density to some extent by improvingthe method for manufacturing the electrodes, but an activated carbonwhich gives electric double layer capacitors showing a high powerdensity and an excellent durability still remains unfound.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an activated carbonsuited for producing an electric double layer capacitor which shows alarge output density per unit volume and which, even when subjected torepeated charge-and-discharge cycles at a large electric current or evenwhen a constant voltage is continuously applied thereto for a longperiod of time, undergoes only a slight decrease in power density.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As a result of intensive investigations to solve the above problems, theinventors have found that activated carbon obtained by subjecting aspecific raw material to an activation treatment under appropriateconditions has an adequate pore distribution, an adequate specificsurface area, an adequate surface state and adequate electrochemicalproperties, and shows a large output density per unit volume and anexcellent durability due to the adequate properties, thus havingachieved the invention based on the finding.

That is, the gist of the invention lies an activated carbon for use inelectric double layer capacitors produced by carbonization of coconutshell, which is characterized in that a BET specific surface area is2000 m²/g to 2500 m²/g, an average pore diameter is 1.95 nm (19.5 Å) to2.20 nm (22 Å) and a pore volume of pores having a pore diametercalculated according to a Cranston-Inkley method of 5.0 nm (50 Å) to30.0 nm (300 Å) is 0.05 cm³/g to 0.15 cm³/g.

As a preferred embodiment of the invention, in the activated carbon foruse in electric double layer capacitors, an oxygen content per g of theactive carbon is 1 mg to 20 mg, a rest potential vs a lithium electrodeis 2.85 V to 3.03 V in a nonaqueous electrolytic solution, and theactivated carbon obtained by subjecting a coconut shell carbonizationproduct to steam activation.

BEST MODE FOR PRACTICING THE INVENTION

The present invention will now be described in more detail below.

The activated carbon of the invention for use in electric double layercapacitors is an activated carbon produced by carbonization of coconutshell, which must satisfy the conditions on the physical properties of aBET specific surface area of 2000 m²/g to 2500 m²/g, an average porediameter of 1.95 nm (19.5 Å) to 2.20 nm (22 Å) and a pore volume ofpores having a pore diameter calculated according to a Cranston-Inkleymethod of 5.0 nm (50 Å) to 30.0 nm (300 Å), of 0.05 cm³/g to 0.15 cm³/gand, preferably, satisfies the conditions on the physical properties onthe physical properties of an oxygen content of 1 mg to 20 mg per g ofthe active carbon, and a rest potential vs a lithium electrode of 2.85 Vto 3.03 V in a nonaqueous electrolytic solution.

In an electric double layer capacitor using the activated carbon of theinvention satisfying the conditions on the physical properties as amaterial for polarizable electrodes, bulk density of the activatedcarbon is enough high, and ion conductivity of electrolyte ion andsolvent molecule of an electrolytic solution existing in the pores ofthe activated carbon is enough large to give a sufficiently high powereven upon charging and discharging under a large electric current. Inaddition, durability of the electric double layer capacitor can be moreimproved by adjusting the amount of oxygen contained in the activatedcarbon to an adequate amount and adjusting a rest potential of theactivated carbon electrode in a nonaqueous electrolytic solution to anadequate range.

Furthermore, the invention can realize such excellent properties of theactivated carbon obtained from coconut shell, and hence the activatedcarbon of the invention has advantages over those obtained fromexpensive synthetic resins such as a phenol resin in productivity andcost.

It is necessary for the activated carbon of the invention for use inelectric double layer capacitors to have a BET specific surface area,determined by the BET method according to a nitrogen adsorption method,of 2000 m²/g to 2500 m²/g, preferably 2000 m²/g to 2400 m²/g, morepreferably 2050 m²/g to 2250 m²/g, because, in case where specificsurface area of the activated carbon is too large, there results adecrease in bulk density, leading to a decrease in power per unitvolume. In case where the specific surface area is smaller than therange, there results a decrease in output per unit weight, thus suchspecific surface area not being preferred.

In addition, it is necessary for the activated carbon of the inventionfor use in electric double layer capacitors to have an average porediameter of 1.95 nm (19.5 Å) to 2.20 nm (22 Å). In case where theaverage pore diameter is too small, electric resistance increases somuch apparently due to diffusion resistance of electrolyte ion withinthe pores upon charging and discharging under a large electric currentthat the resulting capacitors are not suited for uses requiring a highoutput. On the other hand, in case where the average pore diameter istoo large, there results a decrease in bulk density of the activatedcarbon and a decrease in output per unit volume, thus such pore diameternot being preferred. The pore diameter is preferably 2.00 nm (20.0 Å) to2.15 nm (21.5 Å), more preferably 2.02 nm (20.2 Å) to 2.15 nm (21.5 Å).

Further, it is necessary for the activated carbon of the invention foruse in electric double layer capacitors to have a pore volume of poreshaving a pore diameter calculated according to the Cranston-Inkleymethod of 5.0 nm (50 Å) to 30.0 nm (300 Å), of 0.05 cm³/g to 0.15 cm³/g.In case where the pore volume is too large, there results a decrease inbulk density of electrode and a decrease in power per volume, thus suchpore volume not being preferred. And, in case where the pore volume istoo small, there results an increase in inner electrical resistance and,as a result, output density becomes smaller. Thus, the pore volume ispreferably 0.07 to 0.13 cm³/g, more preferably 0.08 to 0.12 cm³/g.

It is necessary for the activated carbon of the invention for use inelectric double layer capacitors to be produced from coconut shell. Ascommon raw materials for activated carbon other than coconut shell,there are illustrated various ones such as petroleum-based pitch,petroleum coke, fibers obtained by spinning tar pitch, syntheticpolymers, phenol resin, furan resin, polyvinyl chloride resin,polyvinylidene chloride resin, polyimide resin, polyamide resin, liquidcrystal polymers, plastic wastes and wasted tires. However, in the caseof using the phenol resin as a raw material for activated carbon, it isdifficult to adjust the average pore diameter and the pore volume ofpores having a pore diameter of 5.0 nm (50 Å) to 30.0 nm (300 Å)respectively to the ranges specified in the invention, though it ispossible to obtain activated carbon having a specific surface area of2000 m²/g or more.

The activated carbon of the invention is obtained by carbonizing coconutshell, then activating the carbonization product. Activating methods areroughly classified into a gas activation method and a chemical agentactivation method. The gas activation method, which is also calledphysical activation in contrast to that the chemical agent activation ischemical activation, is an activation to produce activated carbon bybringing the carbonized raw material into contact with steam, carbonicacid gas, oxygen or other oxidizing gas at a high temperature to conductreaction. The chemical agent activation method is a method of uniformlyimpregnating the raw material with an activating chemical agent, heatingit in an inert gas atmosphere to cause dehydration and oxidationreaction of the chemical agent to thereby obtain activated carbon. Asthe chemical agents to be used, there are illustrated zinc chloride,phosphoric acid, sodium phosphate, calcium chloride, potassium sulfide,potassium hydroxide, sodium hydroxide, potassium carbonate, sodiumcarbonate, sodium sulfate, potassium sulfate and calcium carbonate.

Methods for producing activated carbon are not particularly limited and,so long as the resultant activated carbon satisfies the requirements onthe above-described properties, the methods are not limited to theabove-mentioned methods. Of these activation methods, however, steamactivation method is advantageous since activated carbon obtained bysteam activation method shows an excellent durability when used inelectric double layer capacitors and can be produced at a low productioncost.

As to shape of activated carbon, there are various shapes such aspulverized shape, particulate shape, granular shape, fiber shape, feltshape, woven fabric and sheet, and activated carbon of any of them maybe used in the invention.

The activated carbon in the invention obtained by steam activationmethod is obtained by pulverizing coconut shell, adjusting particle sizeof the pulverized product, then carbonizing (dry distillation) it in aninert atmosphere, and heat-treating the resulting coconut shell char ata temperature of 800° C. to 1300° C., preferably 850 to 1200° C., morepreferably 900 to 1100° C., in an inert gas of nitrogen, argon or acombustion exhaust gas containing a steam gas atmosphere in a content of30% by volume to 100% by volume.

Activated carbons from which metal impurities, ashes, etc. are removedby washing coconut shell before activation treatment, coconut shell charor activated carbon obtained by activation treatment, in an aqueoussolution of hydrochloric acid, nitric acid, sulfuric acid or the like,are also included in the invention.

It is also possible to heat-treat the activated carbon having beensubjected to activation treatment at 500 to 2500° C., preferably 700 to1500° C., in an inert atmosphere of nitrogen, argon, helium or xenon toremove unnecessary functional groups at the surface thereof or enhancingcrystallinity of carbon for increasing electron conductivity.

With particulate activated carbon, the average particle diameter ispreferably 30 μm or less, more preferably 7 to 20 μm, in the point ofimproving bulk density of electrode and reducing its inner electricalresistance.

The activated carbon of the invention for use in electric double layercapacitors shows a rest potential vs Li/Li⁺ as a counter electrode ofpreferably 2.85 V to 3.03 V, more preferably 2.90 to 3.00 V, in anonaqueous electrolytic solution, in the electric double layer capacitorusing the nonaqueous electrolytic solution. In case where thespontaneous potential is too large, there results reduction indurability of an electric double layer capacitor assembled using suchactivated carbon as, for example, a positive electrode, in whichcapacitor electric potential of the positive electrode after charging byapplying a voltage of 2.5 V or more becomes about 4.3 V (vs Li/Li⁺) toreach the oxidation decomposition potential of the electrolytic solution(4.3 V or more), thus decomposition reaction of the electrolyticsolution occurring. Additionally, activated carbons having a spontaneouspotential less than 2.85 V usually cannot be obtained by the aforesaidmethods.

The rest potential of the carbonaceous electrode functioning as positiveelectrode in the invention is measured in a common electrochemicalmanner. Measurement of the potential in a nonaqueous electrolyticsolution is not so strictly defined as with the standard hydrogenelectrode in an aqueous solution. Actually, however, it has widely beenconducted using such electrode as a silver-silver chloride electrode, aplatinum electrode or a lithium electrode. In the invention, too, thepotential can be measured according to the same methods.

Amount of oxygen contained in the activated carbon is a factor exertingsome influence on durability of the electric double layer capacitor and,therefore, it is preferred to control the oxygen amount to an adequateamount. In the invention, the oxygen amount per g of the activatedcarbon is preferably 1 mg to 20 mg, more preferably 2 mg to 10 mg. Inthe invention, the amount of oxygen contained in the activated carbon isa value which is obtained by heat-treating the activated carbon ataround 1000° C. in vacuo or in an inert gas atmosphere of an argon gasor a nitrogen gas, determining carbon monoxide (CO) and carbon dioxide(CO₂) contained in the decomposition gas generated upon the heattreatment, and calculating the total amount of oxygen contained in thesemolecules. Taking into consideration that the thermal decompositiontemperature is around 1000° C., this oxygen amount corresponds to thatof oxygen-containing functional groups in the activated carbon, i.e.,carboxyl group, phenol group, ketone group, and the like and, in otherwords, the oxygen content in the invention can be said to be anindication of the total amount of oxygen-containing functional groupscontained in the activated carbon.

In case where the oxygen amount per g of the activated carbon exceedsthe above-described range, there results an increase in electricalresistance due to generation of gases inside the cell assumed to becaused by decomposition, or reaction with the electrolytic solution, ofthe oxygen-containing functional groups upon charging and discharging ofthe electric double layer capacitor and, as a result, durability of thecapacitor will be decreased, thus such oxygen amount not beingpreferred. On the other hand, in case where the oxygen amount is lessthan the above-described range, there results a decrease in affinity ofthe activated carbon with a binder for electrode upon preparingelectrodes and, as a result, bulk density of the electrode decreases,leading to a decrease in output per unit volume, thus such oxygen amountnot being preferred.

Descriptions are given below with respect to the case of constituting anelectric double layer capacitor using the activated carbon of theinvention.

Polarizable electrodes composed mainly of the activated carbon can beformed in a conventional manner and are mainly constituted by theactivated carbon and a binder and, further, a conductive material may beadded thereto for imparting conductivity to the electrodes. Theactivated carbon can be molded in a conventionally known manner. Forexample, an electrode can be obtained as a molding by addingpolytetrafluoroethylene to a mixture of the activated carbon andacetylene black, mixing them, then press-molding the mixture.Alternatively, an electrode can be obtained as a molding by adding coalpitch having a comparatively high softening point as a binder to theactivated carbon, mixing them, molding the mixture, then calcining themolding to a temperature of the thermal decomposition temperature of thebinder or higher in an inert atmosphere. Further, it is also possible toprepare a polarizable electrode by calcining only the activated carbonwithout using the conductive material and the binder. The electrode maybe formed as a thin coat film, a sheet-like molding or a plate-likemolding, or a plate-like molding composed of a composite material.

Additionally, the activated carbon may be pulverized to a desiredgranular shape before mixing with a binder, or may be mixed as particleshaving a comparatively large particle diameter with a binder, followedby pulverizing together with the binder to obtain a product as a mixtureslurry or a dry pulverization product.

As the conductive material to be used for the activated carbonelectrode, at least one conductive material selected from the groupconsisting of carbon black such as acetylene black and KETJEN BLACK,natural graphite, thermal expansion graphite, carbon fibers, rutheniumoxide, titanium oxide, and metal fibers such as aluminum and nickel. Ofthese, acetylene black and KETJEN BLACK are particularly preferred inthe point that a small amount thereof is enough to effectively improveconductivity. Amount of the conductive material based on the activatedcarbon is preferably 5 to 50% by weight, particularly preferably 10 to30% by weight, though depending upon bulk density of the activatedcarbon, because, in case where the amount of conductive material is toolarge, proportion of the activated carbon is decreased, leading to adecreased volume of the electrode.

As a binder, there is preferably used at least one ofpolytetrafluoroethylene, polyvinylidene fluoride, carboxy cellulose,methyl cellulose, crosslinked fluoro-olefin copolymer, polyvinylalcohol, polyacrylic acid, polyimide, petroleum pitch, coal pitch andphenol resin.

A current collector is not particularly limited, and any one that hasenough corrosion resistance both electrochemically and chemically may beused. For example, as a current collector for a positive electrode, thatmade of stainless steel, aluminum, titanium or tantalum is illustratedand, as a current collector for a negative electrode, that made ofstainless steel, nickel, aluminum or copper is preferably used.

The electrolytic solution is preferably a nonaqueous electrolyticsolution. As solutes of the nonaqueous electrolytic solution, a salt ofa combination of a quaternary onium cation exemplified by R₄N⁺, R₄P⁺(wherein R represents an alkyl group shown by C_(n)H_(2n+1); n=1 to 4)or triethylmethyl ammonium ion and an anion of BF₄ ⁻, PF₆ ⁻, ClO₄ ⁻,SbF₆ ⁻ or CF₃SO₃ ⁻; or a lithium salt wherein cation is lithium ion isused. As the lithium salt, one or more substances selected from amongLiBF₄, LiC1O₄, LiPF₆, LiSbF₆, LiAsF₆, LiCF₃SO₃, LiC(CF₃SO₂)₃,LiB(C₆H₅)₄, LiC₄F₉SO₃, LiC₈F₁₇SO₃ and LiN(CF₃SO₂)₂ are preferred. Inparticular, in view of conductivity, stability and low cost, salts of acombination of a cation of R₄N⁺ (wherein R represents an alkyl groupshown by C_(n)H_(2n+1); n=1 to 4) or triethylmethyl ammonium ion and ananion of BF₄ ⁻, PF₆ ⁻, ClO₄ ⁻ or SbF₆ ⁻ are preferred.

Concentration of the solute in the nonaqueous electrolytic solution ispreferably 0.3 to 2.0 mols/liter so as to sufficiently obtaincharacteristic properties of the electric double layer capacitor, with0.7 mol/liter to 1.9 mols/liter being particularly preferred to obtain ahigher conductivity. A concentration of not less than 2.0 mols/liter isnot preferred because, upon charging and discharging at a lowtemperature of −20° C. or lower, conductivity of the electrolyticsolution is decreased. A concentration of not more than 0.3 mol/liter isnot preferred because conductivity becomes small at room temperature orat lower temperatures.

As the electrolytic solution, a solution of tetraethylammoniumtetrafluoroborate (Et₄NB₄) in propylene carbonate is preferred, andconcentration of Et₄NB₄ is preferably 0.5 to 1.0 mol/liter.

Solvents for the nonaqueous electrolytic solution are not particularlylimited, but are preferably one or more organic solvents selected fromamong propylene carbonate, ethylene carbonate, butylene carbonate,dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate,sulfolane, methylsulfolane, γ-butyrolactone, γ-valorolactone,N-methyloxazolidinone, dimethylsulfoxide and trimethylsulfoxide. In viewof excellent electrochemical and chemical stability and conductivity,one or more organic solvents selected from among propylene carbonate,ethylene carbonate, butylene carbonate, dimethyl carbonate, methyl ethylcarbonate, diethyl carbonate, sulfolane, methylsulfolane andγ-butyrolactone are particularly preferred. However, high-melting pointsolvents such as ethylene carbonate cannot be used independently becausethey become solid at lower temperatures, and must be used as a mixedsolvent with low-melting point solvents such as propylene carbonate orthe like.

Water content in the nonaqueous electrolytic solution is preferably 200ppm or lower, more preferably 50 ppm or lower, so as to obtain a highwithstand voltage.

EXAMPLES

The invention is now described in more detail by reference to specificexamples, but is not limited thereto unless exceeding the gist of theinvention.

Examples 1 to 6, Comparative Examples 1 to 5

Example of Producing Activated Carbon

Coconut shell char (50 g) was placed in a rotary kiln into which asteam-containing nitrogen gas was allowed to pass, to thereby conductsteam activation. Activation temperature, activation time andconcentration of steam in the nitrogen gas were as shown in thefollowing Table-1. The thus obtained activated product was washed withhydrochloric acid, and repeatedly washed with demineralized water. Afterwashing, the activated product was dried, and the dried activatedproduct was pulverized to obtain active carbon powder having an averageparticle diameter of 10 to 20 μm (Examples 1 to 6, Comparative Examples1, 3) In Comparative Example 2, the same procedure as in Example 1 wasconducted except for using a pulverized product of bituminous coal inplace of the coconut shell char to obtain activated carbon powders and,in Comparative Examples 4, 5, the same procedure as in Example 1 wasconducted except for respectively using carbonized products obtained bycarbonizing phenol resin (novolak) cured product at 700° C. in anitrogen atmosphere in place of the coconut shell char to obtainactivated carbon powders.

TABLE 1 Steam Activation Activation Concentration Temperature Time (vol%) ° C. min. Example 1 80 930 135 2 80 930 125 3 60 900 150 4 60 900 1355 60 900 130 6 60 900 125 Com. Ex. 1 60 900  80 2 60 900 140 3 60 900130 4 60 900 140 5 60 900 130Measurement of Physical Properties of Activated Carbon

BET specific surface area, pore volume of pores having a diameter of 5.0nm (50 Å) to 30.0 nm (300 Å), and total pore volume, of activated carbonpowders thus obtained were calculated from adsorption isotherm ofactivated carbon obtained by measuring equilibrium adsorption amount ofnitrogen gas of each activated carbon powder under each relativepressure at a liquid nitrogen temperature using a Sorptomatic Model 1800(made by Faison).

Average pore diameter of the activated carbon powder was calculated fromthe total pore volume and the BET specific surface area determined fromthe above-described isotherm of nitrogen gas assuming that the pore ofthe active carbon is of a cylindrical shape.

Amount of oxygen contained in the activated carbon was determined asfollows.

A quartz glass-made reaction tube containing therein about 1 g ofactivated carbon powder was heated to about 1000° C., and a gasgenerated during the heating was injected into a gas chromatographanalyzer to determine a CO component and a CO₂ component in thegenerated gas. Sum of the oxygen amounts of determined CO and CO₂components was calculated to obtain the amount of oxygen contained per gof the activated carbon.

Physical properties of the activated carbons thus calculated aretabulated in Table-2.

TABLE 2 (Physical Properties of Active Carbon) Specific Total Average5.0–30.0 Amount of Surface Pore Pore nm Contained Spontaneous Raw AreaVolume Diameter volume Oxygen Potential Material m²/g cm³/g nm cm³/gmg/g V vs Li/Li⁺ Example 1 Coconut 2351 1.20 2.03 0.106 8.1 3.02 shell 2Coconut 2294 1.14 2.00 0.090 6.1 3.02 shell 3 Coconut 2250 1.14 2.030.075 8.5 3.01 shell 4 Coconut 2154 1.08 2.01 0.126 9.1 3.02 shell 5Coconut 2147 1.07 2.00 0.121 1.8 2.99 shell 6 Coconut 2024 1.00 2.000.130 2.5 2.99 shell Com. Ex. 1 Coconut 1589 0.74 1.87 0.035 1.9 2.98shell 2 Coal 1387 0.73 2.10 0.666 9.2 3.02 3 Coconut 1960 0.98 2.010.122 21 3.05 shell 4 Phenol 2110 0.944 1.79 0.021 2.9 3.03 resin 5Phenol 1855 0.808 1.74 0.010 2.3 3.02 resin

Test Example 1

(Evaluation-1 of Capacitor Performance of Activated Carbon: InitialPerformance)

Each of the activated carbons obtained in the above-described examplesof producing activated carbons was tested as follows. A mixture composedof 80% by weight of the activated carbon powder, 10% by weight ofacetylene black and 10% by weight of polytetrafluoroethylene waskneaded, and press-molded into a disc-like molding of 10 mm in diameterand 0.5 mm in thickness under a pressure of 50 kgf/cm² through an oilhydraulic press using a tablet-molding apparatus (made by Nippon BunkoCo., Ltd.), thus a positive electrode molding being obtained. Similarly,another sheet of the molding was prepared to use as a negative electrodemolding. The thus obtained two moldings were dried at 300° C. for 3hours under a vacuum of 0.1 torr or lower. The thus dried two moldingswere transferred into a dry box kept in a nitrogen gas atmosphere, thenthe positive and negative electrode moldings were impregnated with anelectrolytic solution of a solution of (C₂H₅)₄NBF₄ in propylenecarbonate in vacuo. Subsequently, the positive electrode molding and thenegative electrode molding impregnated with the electrolytic solutionwere disposed in confronting relation to each other with apolyethylene-made separator interposed therebetween, followed bycaulk-sealing the assembly inside a stainless steel-made coin cell toobtain a coin cell type electric double layer capacitor.

To the thus obtained coin cell type electric double layer capacitor wasapplied a voltage of 2.5 V in a 25° C. thermostatic chamber using acharge-and-discharge tester, model HJ-201 (made by Hokuto Denko Co.,Ltd.), then the capacitor was allowed to discharge at a constant currentof 20 mA/cm² in current density. Initial electrostatic capacity (F) wasdetermined from the slope of the thus obtained discharge curve between2.5 V and 1.0 V. In addition, voltage drop immediately after discharge(IR-drop) was read off from the discharge curve. A too large IR-drop isnot preferred, since only a small energy density is eventuallyavailable. Results are shown in Table-3.

TABLE 3 (Initial performance 25° C./2.5 V coin cell) ElectrostaticCapacity IR-drop (F.) (V) Example 1 0.96 0.05 2 0.98 0.10 3 1.02 0.12 40.97 0.13 5 1.03 0.10 6 1.00 0.13 Comparative Example 1 0.90 0.25 2 0.880.26 3 0.90 0.30 4 0.88 0.33 5 0.88 0.34

Test Example 2

(Evaluation-2 of Capacitor Performance of Activated Carbon: InitialPerformance)

Distilled water was added to a mixture composed of 8 parts by weight byeach of the activated carbons obtained in the examples of producing theactivated carbon, 3 parts by weight of conductive carbon black and 3parts by weight of cellulose series binder, then kneading the mixture toobtain a coating paste of an electrode. The thus obtained paste wascoated on an etched aluminum foil, followed by drying to obtain anelectrode body having coated thereon a 40-μm thick activated carbonpaste. From this electrode body were obtained two electrode bodieshaving an effective electrode area of 7.07 cm×7.07 cm (50 cm²), onebeing used as a positive electrode and the other as a negativeelectrode. The positive and negative electrodes were disposed inconfronting relation to each other with the activated carbon electrodecoat inside, with a cellulose series separator being providedtherebetween, to obtain an electrochemical element. This element wassandwiched between glass plates, and the glass plates were sandwichedbetween stainless steel-made plates, followed by fixing the elementusing bolts and nuts to obtain an electric double layer capacitorelement. The thus obtained capacitor element was heat-dried in vacuo toremove impurities. Subsequently, the element was impregnated with anelectrolytic solution of a solution of (C₂H₅)₄NBF₄ in propylenecarbonate to obtain an electric double layer capacitor.

To the thus obtained electric double layer capacitor was applied avoltage of 2.5 V in a 25° C. thermostatic chamber using a commerciallyavailable charge-and-discharge tester, then the capacitor was allowed todischarge. Electrostatic capacity (F/cm³), inner electrical resistance(Ω) and power density (W/cm³) obtained by discharging from 2.5 V to 1.5V for 2 seconds were calculated from the discharge curve. Capacitorperformances per unit volume (F/cm³ and W/cm³) were calculated byconverting the electrostatic capacity (F) and output (W) determined fromthe obtained discharge curve to values per volume of the activatedcarbon electrode coat. The thus calculated capacitor properties areshown in Table-4.

TABLE 4 (Initial properties of capacitors, 25° C./2.5 V) Inner 2-Sec.Electrostatic Electrical Power Capacity Resistance Density F/cm³ Ω W/cm³Example 3 13.0 0.15 7.8 Example 4 12.2 0.14 7.4 Example 5 1.00 0.14 7.7Comparative 12.1 0.19 6.8 Example 1 Comparative 9.7 0.15 6.6 Example 2

Test Example 3

(Evaluation-3 of Capacitor Performance of Activated Carbon: Durability)

Durability test of the electric double layer capacitors was conducted inthe following manner.

A mixture composed of 80% by weight of the activated carbon powder ofeach of the activated carbons obtained in the above-described examplesof producing the activated carbon, 10% by weight of acetylene black and10% by weight of polytetrafluoroethylene was kneaded, and press-moldedinto a disc-like molding of 10 mm in diameter and 0.5 mm in thicknessunder a pressure of 50 kgf/cm² through an oil hydraulic press using atablet-molding apparatus (made by Nippon Bunko Co., Ltd.), thus apositive electrode molding being obtained. Similarly, another sheet ofthe molding was prepared to use as a negative electrode molding. Thethus obtained two moldings were dried at 300° C. for 3 hours under avacuum of 0.1 torr or lower. The thus dried two moldings weretransferred into a dry box kept in a nitrogen gas atmosphere, then thepositive and negative electrode moldings were impregnated with anelectrolytic solution of a solution of (C₂H₅)₄NBF₄ in propylenecarbonate in vacuo. Subsequently, the positive electrode molding and thenegative electrode molding impregnated with the electrolytic solutionwere disposed in confronting relation to each other with apolyethylene-made separator interposed therebetween, followed bycaulk-sealing the assembly in a stainless steel-made coin cell to obtaina coin cell type electric double layer capacitor.

To the thus obtained coin cell type electric double layer capacitor wasapplied a voltage of 2.8 V in a 70° C. thermostatic chamber using acharge-and-discharge tester, model HJ-201 (made by Hokuto Denko Co.,Ltd.), then the capacitor was allowed to discharge at a constant currentof 10 MA/cm² in current density. Initial electrostatic capacity (F) wasdetermined from the slope of the thus obtained discharge curve. Then,after continuously applying a voltage of 2.8 V for 500 hours, thecapacitor was allowed to discharge, and electrostatic capacity (F) afterdurability test was determined. Change of the capacity in terms of theratio of electrostatic capacity after durability test to the initialcapacity (%) are shown in Table-5.

TABLE 5 (durability, 70° C./2.8 V) Initial After Rate of Capacity 500hrs Change F F % Example 2 1.50 1.38 92 Example 3 1.57 1.43 91 Example 41.49 1.37 92 Example 6 1.49 1.39 93 Comparative 1.47 1.15 78 Example 3

Test Example 4

(Measurement of Spontaneous Potential of Activated Carbon)

A mixture composed of 80% by weight of the activated carbon powder ofeach of the activated carbons obtained in the examples of producing theactivated carbon, 10% by weight of acetylene black and 10% by weight ofpolytetrafluoroethylene was kneaded, and press-molded into a disc-likemolding of 10 mm in diameter and 0.5 mm in thickness under a pressure of50 kgf/cm² through an oil hydraulic press using a tablet-moldingapparatus (made by Nihon Bunko Co., Ltd.). The thus obtained molding wasdried at 300° C. for 3 hours under a vacuum of 0.1 torr or lower. Thethus dried activated carbon electrode and a metallic lithium foil of 0.5mm in thickness punched as a 10-mm diameter piece were disposed inconfronting relation to each other with a polyethylene-made separator(made by Mitsubisi Chemical Corporation) interposed therebetween,followed by sandwiching the activated carbon electrode and the metalliclithium foil from outside by platinum plates functioning as a currentcollector. The resulting assembly was then held from the outermost sidesby two sheets of 5-mm thick Teflon plate having four bolt holes tosecurely contact the current collectors, the activated carbon electrodesand the separator to each other. This was then dipped in a 1 mol/litersolution of LiBF₄ in propylene carbonate placed in a beaker.Subsequently, the current collector on the activated carbon electrodeside and the current collector on the metallic lithium foil side werewired to each other via a potentiometer to measure a spontaneouspotential of the activated carbon electrode. Spontaneous potentials ofrespective activated carbon electrodes in case of using, as a counterelectrode thereof, lithium (V vs Li/Li⁺) are shown in Table-2.

Industrial Utility

An electric double layer capacitor which can give a large power densityper unit volume and which, even when charge-and-discharge cycle isrepeated under a large current or a constant voltage is continuouslyapplied for a long time, undergoes less decrease in power density can beprovided by using activated carbon of the invention manufactured fromcoconut shell and having an adequate pore distribution, an adequatespecific surface area, an adequate surface state and adequateelectrochemical properties.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

This application is based on Japanese patent application No. 2000-136253filed on May 9, 2000, the entire contents thereof being herebyincorporated by reference.

1. An activated carbon suitable for use in electric double layer capacitors, said activated carbon being produced by carbonization of a carbonaceous material consisting essentially of coconut shell, wherein said activated carbon is produced by a process comprising: pulverizing a coconut shell; carbonizing said coconut shell in an inert atmosphere to produce a coconut shell char; and heat-treating said coconut shell char at a temperature ranging from 900° C. to 1,100° C. in a steam gas atmosphere containing an inert gas selected from the group consisting of nitrogen, argon, and a combustion exhaust gas, wherein the content of steam in said steam gas atmosphere ranges from 30% by volume to 100% by volume, wherein said activated carbon has a BET specific surface area of 2000 m²/g to 2500 m²/g, and an average pore diameter of 1.95 nm (19.5 Å) to 2.20 nm (22 Å), wherein the pore volume of pores having a pore diameter, as calculated according to a Cranston-Inkley method, of 5.0 nm (50 Å) to 30.0 nm (300 Å) is 0.05 cm³/g to 0.15 cm³/g, wherein the amount of oxygen contained per g of said activated carbon is 1.8 mg to 8.1 mg, and wherein said activated carbon exhibits a spontaneous potential versus a lithium electrode of 2.85 V to 3.03 V in a non-aqueous electrolytic solution.
 2. The activated carbon for electric double layer capacitors as claimed in claim 1, wherein the BET specific surface area is 2000 m²/g to 2400 m²/g.
 3. The activated carbon for electric double layer capacitors as claimed in claim 1, wherein the BET specific surface area is 2050 m²/g to 2250 m²/g.
 4. The activated carbon for electric double layer capacitors as claimed in claim 1, wherein the pore volume of pores having a pore diameter, calculated according to a Cranston-Inkley method, of 5.0 nm (50 Å) to 30.0 nm (300 Å) is 0.07 cm³/g to 0.13 cm³/g.
 5. The activated carbon for electric double layer capacitors as claimed in claim 1, wherein the pore volume of pores having a pore diameter, calculated according to a Cranston-Inkley method, of 5.0 nm (50 Å) to 30.0 nm (300 Å) is 0.08 cm³/g to 0.12 cm³/g.
 6. The activated carbon for electric double layer capacitors as claimed in claim 1, wherein the average pore diameter is 2.00 nm to 2.15 nm.
 7. The activated carbon for electric double layer capacitors as claimed in claim 1, wherein the average pore diameter is 2.02 nm to 2.15 nm.
 8. The activated carbon for electric double layer capacitors as claimed in claim 1, wherein an oxygen content per g. of the activated carbon is 2.5 mg to 6.1 mg.
 9. The activated carbon for electric double layer capacitors as claimed in claim 8, wherein said pore volume thereof is from 0.08 cm³/g to 0.12 cm³/g.
 10. The activated carbon for electric double layer capacitors as claimed in claim 1, having a specific surface area of from 2024–2351 m²/g.
 11. The activated carbon for electric double layer capacitors as claimed in claim 1, having a total pore volume of 1.00–1.20 cm³/g.
 12. The activated carbon for electric double layer capacitors as claimed in claim 1, having an average pore diameter of 2.00–2.03 nm.
 13. The activated carbon for electric double layer capacitors as claimed in claim 1, having a pore volume of pores having a 5.0–30.0 nm diameter of from 0.075–0.130 cm³/g.
 14. The activated carbon for electric double layer capacitors as claimed in claim 1, having a spontaneous potential of 2.99–3.02.
 15. An electric double layer capacitor, comprising the activated carbon of claim
 1. 